The target of the AIM mission is asteroid 65803 Didymos (1996 GT), an
Apollo-type near-Earth object (NEO) with a perihelion (minimum distance
to the Sun) that is just below the aphelion radius (maximum distance to
the Sun) of Earth orbit. Didymos is a binary asteroid; the primary body
has a diameter of around 775 m and a rotation period of 2.26 hours, whereas
the secondary body (informally called Didymoon) has a diameter of around
165 m and rotates around the primary at a distance of around 1.2 km in
around 12 hours.

In 2015 telescopes around the globe homed in on the Didymos system to
study the shape and size of the double asteroid and try to ascertain the
orbital pole of Didymos and refine the shape model of the primary, among
other things, to help plan ESA's AIM mission proposal, which is part
of the AIDA project.

The orientation of Didymoon's orbit with respect to the ecliptic is
one of the key factors involved in planning the AIM/AIDA mission. It is
important, for example, in calculating the approach of the spacecraft
to the binary system, as well as to know when the asteroid moon will fall
into shadow - needed for scheduling payload operations. The information
obtained during the latest observation period was put together with already
known data to refine the orbital pole of the primary, its shape and other
dynamical parameters, putting constraints on the bulk density of the two
bodies.

A preliminary shape model of the Didymos primary could be extracted from
previous radar observations in combination with new lightcurve data.

When the AIM spacecraft has arrived at the binary system it will collect
extensive amounts of data on both the primary and secondary asteroid,
including precise measurements of the asteroid's mass and shape and
thermal imaging of the surface to understand the structure and mineralogical
composition of the soil.

Thermal imaging

A variety of measuring techniques will be available through the instruments
on board AIM and its lander/cubesats (see Payload) to comprehensively
detail the Didymos system before DART, the impactor, will hit Didymoon.
In particular, direct information on the internal structure of an asteroid
will be obtained for the first time. AIM will watch the impact and will
again collect masses of data after impact to be able to calculate the
effect of the impact on both the shape, structure and orbit of Didymoon
around the primary.

The thorough study of Didymoon and its impact by DART will be the first
fully documented impact experiment at asteroid scale and will offer valuable
insights into the origin and history of our Solar System that need a better
understanding of the impact process at large scales. It will also provide
scientists invaluable data to develop and assess the efficiency of planetary
defense strategies against any incoming asteroids in the future.

Despite Didymos' close encounter with the Earth in 2022, this asteroid
poses no threat as there is no risk of impact. It is therefore the ideal
candidate to run an asteroid mitigation precursor experiment whose objective
is to change the orbit of Didymoon around its primary main body thus leaving
the binary asteroid's path around the Sun unchanged. Such experiment will
give us crucial information about whether the kinetic impactor technique
would work to deflect an asteroid if ever one was discovered to be headed
our way.
Received on Tue 16 Feb 2016 01:19:25 AM PST